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Superheat or not?

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Perko717/08/2019 08:06:48
278 forum posts
23 photos

General chinwag among club members over a cuppa at a working bee today generated some discussion on whether superheaters are any advantage in a typical 5inch gauge loco. The consensus amongst the 'knowledgeable' was that they were a waste of time and potentially dangerous. An example was given of an over-filled boiler which ended up with water passing through the regulator into the superheater which acted as a flash steam generator making the loco unstoppable until all the water had evaporated. I note from early ME's that LBSC was 100% in favour of them. I'd be interested to know what current practice is in other parts of the world.

fizzy17/08/2019 09:05:39
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1602 forum posts
106 photos

There is a thread on here somewhere which goes into great detail on this subject, with lots of for and against views. The upsum of the thread was that they arent essential but the boiler will use less fuel and make more steam with them used. But there are good superheaters and bad ones...another story.

SillyOldDuffer17/08/2019 10:08:31
4601 forum posts
988 photos
Posted by fizzy on 17/08/2019 09:05:39:

... But there are good superheaters and bad ones...another story.

Fizzy's comment is spot on I think.

Though I'm never likely to build a boiler, I've always been interested in how they work. In theory superheat is without doubt a good thing, and - from about 1890 - full-size locomotives adopted it almost universally. As well as being more fuel efficient superheat also makes the engine more powerful.

Superheaters don't scale down well. The problem is small engines leak heat far faster than a big one. One reason is the ratio of surface area to boiler volume increases rapidly as diameter of the boiler reduces. Another is full-size boilers have space for a few inches of cladding, while little one's don't. Same issue with pipes, valves, and cylinders - all waste heat. The first problem with superheating a small engine is ensuring superheated steam actually reaches the pistons in enough volume to make a difference. Not easy.

A second problem is wet steam acts as a lubricant helping pistons and valves to run freely. Superheat dries the steam out removing this benefit. I think it likely that an engine might have enough superheat to lose the lube effect whilst not having enough heat to improve performance. The effects balance out.

I don't believe water splashing into a superheater tube would make enough steam to make an engine uncontrollable. Anyone seen a runaway happen in practice?

If you fancy being controversial, tell the 'knowledgeable' that they're completely wrong. Superheating does work, it's just too difficult for bad builders!

More helpfully, I suspect the layout may be more critical than builder skills. A design than conserves heat by minimising the length of passages and keeping the cylinders warm might take superheat better than one with long or exposed steam surfaces. Just a thought, but LBSC specialised in engines that ran rather than being accurate models of prototypes. Possibly his designs are a bit more superheater friendly than others?

There's a parallel between the failure of superheating on small engines and compounding. In theory compounding is also 'a good thing'. In practice, so much heat is lost in the first cylinder there isn't enough left to effectively drive a second. Due to severe shortage of space, compounding on full-size locomotives hasn't worked well either - the extra complexity doesn't deliver enough benefit. Different story on marine steam engines where relaxed space and weight limits meant engineers could work with much higher boiler pressures and layout efficient triple or quadruple expansion engines.

Dave

Paul Kemp17/08/2019 21:13:52
297 forum posts
11 photos

Dave,

i have been on a full size loco fitted with superheaters that had water carry over through the level being too high and that ran away or rather carried on when the regulator was closed. Solution was to wind into mid gear and it stopped but there was still pressure on the steam chest so yes it can happen. Not sure on a miniature it would run very far though, as you say the volumes are small but still in proportion I guess. Any driver that couldn't deal with this is not much of a driver though unless it was pole reverse in which case he may need a bit of help to pull it back to mid gear from the fireman!

Paul.

duncan webster18/08/2019 17:50:35
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2204 forum posts
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Time for some sums. To convert 1 kg of water at 20C to saturated steam at 6Bara (about 75 psig) takes 2672 kJ,m to then superheat it by 100C takes 201 kJ. The superheater contributes only about 1/12 of the heat transfer, and on its own would be very poor at making steam to drive a loco. However, if you managed to fill the superheater with water from the boiler, close the regulator and then allow this water to flash off into steam at say 2Bara (about 15 psig), you would finish up with water and steam at lower temperature. My sums say 75 gms of steam and 925 gms of water.

The superheater elements on one of my my locos would contain only 60 gm of water, so we could produce 4.5 gm of steam, which would occupy 3985 cc. Cylinders are 1.5" * 2", so one rev is 232cc. I could therefore do a max of 17 revs on this flashed off steam. This is very much top end, as initially the steam would be flashing off at higher pressure and density so would be used up more quickly. In reality if the water level gets too high in my boiler it starts hydraulicing, and power is seems to be reduced

There appears to be a mindset amongst some model engineers that superheat is a bad thing. The theory says it reduces coal and water consumption, Bill Hall's measurements say the same, and this afternoon I spoke to a prolific builder/driver of locos at our club who is of the same opinion, when he replaced the firetube superheaters on one of his locos with radiant there was a noticeable reduction in water consumption. If you are using less water you will use less coal. At a given steam chest pressure and cutoff superheat will NOT increase power output, but for the same coal consumption you can drive a superheated loco harder

Stephan Gaal16/09/2019 06:15:08
4 forum posts

Regardless of all the sums the biggest advantage of super heaters on a model loco is that the steam coming out the stack is hotter and less likely to create a big white cloud in front of you that makes life difficult, especially if you wear glasses. This I discovered by accident when my super heaters failed and I did a quick patch up to bypass them so I could run. After that a new set was soon made.

Do they save fuel and water ? Perhaps but half the fun of steam is shoveling coal so I don't mind either way.

SillyOldDuffer16/09/2019 19:07:53
4601 forum posts
988 photos
Posted by duncan webster on 18/08/2019 17:50:35:

...

At a given steam chest pressure and cutoff superheat will NOT increase power output, but for the same coal consumption you can drive a superheated loco harder

Um, can you explain a bit more please Duncan? I may be misunderstanding what you said, or the thermodynamics or both. Again!

My view. As a loco is a heat engine, isn't its achievable power-output related to the amount of heat in the cylinder rather than simply to steam pressure?

If pressure alone determined power output then an engine with a boiler full of compressed air at 100psi would be as effective as one with steam at 100psi. Is that so? I don't believe a cylinder full of air can do as much work as the same volume of steam at the same pressure, even if the air is at the same temperature.

Air (has pressure but not much heat) is good for turning engines over, but not for doing work. Steam has pressure and heat and contains a lot more energy.

Dave

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